British scientists have been given the green light to genetically modify human embryos, for the first time in the nation's history.

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The landmark decision means scientists will now be allowed to alter the DNA of embryos, for research purposes only.

It remains illegal for these genetically altered embryos to be implanted in a woman. It is hoped the experiments will improve our understanding of the earliest stages of embryo development.

The research, which was approved by the Human Fertilization and Embryology Authority, will use excess embryos donated by couples who have had in vitro fertilization treatment.

Scientists will be focusing on the first seven days of a fertilized egg's growth. In these early days, a fertilized egg evolves from a single cell to around 250 cells.

The research, which will be led by Dr. Kathy Niakan, will take place at the Francis Crick Institute in London and has been hailed as a "triumph for common sense" by leading figures of the British science community.

"The decision allows basic scientific research into early embryo development and miscarriage to continue."

However, the research has also raised concerns that it could pave the way for "designer babies" -- going beyond health improvements and modifying everything from a child's eye color to intelligence.

"I am absolutely certain this is coming," Ronald Green, a Dartmouth College professor and author of "Babies by Design: The Ethics of Genetic Choice," told CNN. He has served on the National Institutes of Health's human embryo research panel.

"By the end of this century, I am absolutely confident that we will have the tools for someone with the means to use this information to change the child they can have through this process."

It is not the first time a country has genetically modified human embryos. In April of last year, scientists in China became the first in the world to edit a gene that causes a blood disorder.

That same month, the U.S. National Institutes of Health released a statement saying it would not fund gene editing technologies in human embryos.

"The concept of altering the human germline in embryos for clinical purposes has been debated over many years from many different perspectives, and has been viewed almost universally as a line that should not be crossed," NIH Director Francis Collins said.

The UK program is set to begin in the next few months, subject to ethical approval.